DIFFERENTIAL COOLING OF A PLUTON EMPLACED INTO CRUST WITH A LATERAL THERMAL GRADIENT: INSIGHTS FROM ALKALI-FELDSPAR CRYSTAL SIZE DISTRIBUTIONS ACROSS THE CENTRAL WHITNEY PLUTON, SIERRA NEVADA, CALIFORNIA

The 83.5 Ma Whitney pluton is the youngest member of the concentrically nested Mount Whitney Intrusive Suite in the eastern Sierra Nevada. Along its northeastern and western margins the Whitney pluton was emplaced into the slightly older Paradise pluton (85 Ma). Along its southeastern margin, however, it was emplaced into older plutonic and metamorphic rocks. This difference between younger, warmer wall rock on the west and older, cooler wall rocks on the east probably created a significant lateral thermal gradient across the central and southern parts of the growing Whitney pluton. The size distributions of alkali-feldspar crystals also differ markedly across the central part of the pluton. Alkali-feldspar megacrysts are smaller and less abundant - and groundmass crystals conversely larger and more abundant - near the pluton’s eastern margin than they are in either its interior or along its western margin.

In a study of the size distributions of alkali-feldspar crystals in the Cathedral Peak pluton, Higgins (Geological Society Special Publication 168, 1999) proposed that megacrysts develop in granitic magmas as a result of textural coarsening when temperatures are held just below the saturation temperature of alkali-feldspar for extended periods of time. Under these conditions, large crystals grow at the expense of their smaller neighbors that have higher surface energy to volume ratios. Applying this model to the asymmetric crystal size distributions of the central Whitney pluton suggests that magmas intruded along its eastern margin cooled relatively rapidly against older wall rocks so that significant coarsening did not occur, whereas those intruded along its warmer western margin cooled more slowly and so developed larger, more abundant megacrysts.

Ongoing study of alkali-feldspar size distributions in the Whitney pluton is directed towards learning if (1) measured separations between megacrysts can be modeled by diffusion-limited growth to yield estimates of the cooling rates of different parts of the pluton; and (2) if these rates can, in turn, can be reproduced by conductive cooling models to constrain the volume rate of the pluton’s growth.